1. Field of the Invention
The present invention relates to a low surface distortion optical device, such as a monochromator and, more particularly, to the specific structural arrangement used to achieve low surface distortion when the surface is heated.
Description of the Prior Art
The purpose of a monochromator is to selectively defract a narrow band of energy, for example, X-rays, from a broadband source and reject undesired wavelengths. This is accomplished by using the lattice structure of silicon crystals as the defracting medium. Any distortion of the silicon lattice tends to smear the defracted waves. In one application, typical synchrotron sources, only approximately the upper 100 microns of the silicon surface function to defract the desired wavelengths. The X-rays that are not defracted are absorbed by the silicon as heat.
Excessive thermal buildup in the silicon can result in thermal distortion of the monochromator, which may be caused by silicon expansion per se, or by bending resulting from uneven expansion of the materials of the monochromator.
Referring to FIG. 1, there is a schematic illustration of a double crystal monochromator, wherein a first crystal 10 is placed in a high-power X-ray beam 16. Ideally, the beam is received at an acute angle .theta.1 and is reflected from the surface of the crystal 10 at a slightly different angle .theta.2. The reflected beam then impinges on a second crystal 12 to be reflected to a using device 14.
Due to excessive thermal buildup, expansion or warping of the first crystal 10 may take place, as shown in FIG. 1, particularly where the beam 16 impinges on the crystal surface. The surface distortion gives rise to defracted rays dispersed in both wavelength and direction, so that they can no longer simultaneously satisfy the Bragg condition for a particular angular setting of the second crystal 12. Hence, only a fraction of the beam's cross-section is defracted by the second crystal, resulting in a loss of throughput.
A monochromator is particularly sensitive to the thermal buildup, since the beam impinges upon the monochromator surface at an angle, so that the beam distribution on the surface is uneven, as shown in FIG. 2, which illustrates the surface of the monochromator 10 that is illuminated by an incoming beam 16.
Referring to FIG. 3, there is shown a three-dimensional plot of the power density of the beam as it hits the surface. The heat that may be absorbed by the monochromator is proportional to the power density. In FIG. 3, power density is plotted along the Y-axis in kilowatts/(milliradian).sup.2, the X-axis represents angular space in beam cross-section by beam longitudinal dispersion (milliradians).sup.2, and the Z-axis represents angular space in beam height by beam longitudinal dispersion (milliradians).sup.2.
As is evident from FIG. 3, the surface heating of the monochromator is extremely uneven, and quickly leads to undesired surface distortion.
Prior art devices have concentrated on cooling the silicon crystal, with a cooling system and geometry designed not only to remove heat but also to minimize the thermal deformation.
Various systems have been developed for cooling monochromators and other optical devices which absorb high-energy radiation. An accepted way for cooling these devices is through the use of internal cooling, wherein the device is mounted on a cooling manifold for distributing a coolant in proximity to the optical device for providing the required cooling. In order to enhance the cooling efficiency, a turbulent coolant flow was often desired.
Optical devices such as monochromators generally include an optical faceplate having an exposed surface for receiving the required radiation. In most instances, as previously mentioned, the active portion of the faceplate material lies within approximately 100 um of the exposed faceplate surface; therefore, any thickness of the faceplate over about 100 um is considered to be excess and may be needed only for structural purposes. In order to provide the most efficient cooling possible, the active material should be in close proximity to the coolant; therefore, it is desirable that the faceplate be made as thin as possible. Most prior art devices have had a relatively thick faceplate on the order of 2 mm or larger.
While it is desirable to have a thin faceplate, this desirable characteristic results in additional problems, in that the thin faceplate is structurally weak and therefore may be adversely affected by the pressure and flow characteristics of the coolant. High coolant pressures can result in a bowing or bending of the faceplate. A turbulent flow of the coolant can generate vibrations in the faceplate. Any displacement or movement of the faceplate surface will distort the resulting radiation.
It has become customary to provide small cooling channels along a surface of a coolant manifold, with said surface being covered by the faceplate, so that the faceplate essentially forms one wall of the cooling channels. This structure provides the beneficial result that the coolant is in direct contact with the faceplate material, but again subjects the faceplate to coolant pressure and vibration due to turbulent coolant flow, thereby necessitating a thick faceplate for structural purposes. The thick faceplate impedes the heat flow from the optical surface to the coolant.
Most prior art devices have either paid little attention to controlling the coolant flow in each channel, or have micro-managed the flow such that each coolant channel has its own inlet and outlet and flow control means. The former approach results in a simple design but rather uneven cooling that can result in unacceptable thermal distortions in the device. The latter approach is extremely complicated, requiring flow control for each micro-channel, resulting in excessively high cost.
In order to reduce the coolant pressure and the possible distortion on the faceplate, some prior art systems have used a dual pumping arrangement so that the coolant pressure may be reduced without a diminution of the coolant flow rate.